There is no issue or no change regarding upload feature in FileVista 7.9.3. So you say it happens only on Firefox and I found this:
This issue can be caused by corrupted cookies.
Clear the cache and cookies only from websites that cause problems.
"Clear the Cache":
- Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"
"Remove Cookies" from sites causing problems:
- Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"